This project set out to investigate cellular responses to subthreshold challenges of trace contaminants in the environment. This project has demonstrated that a cell responds to a subthreshold challenge of cadmium ion by developing a protective mechanism against subsequent suprathreshold exposure. A study of the physiology of this protective response showed that the level of protection is dependent upon the concentration of the first exposure, that protection can raise the threshold by one order of magnitude, that protection is elicited when initial exposure occurs within the greater part of the cell cycle, that the development of protection is time-dependent, and that protection persists for at least a few generations. Other divalent cations were tested in this system for subthreshold responses against cadmium and by cadmium. Three grouping were observed: Protection, potentiation and no response. These observations point to the need to exercise caution in assessing the effects of single, and cumulative, low-level stressors in the environment. Biochemical studies are currently in progress to identify the bases for cadmium toxicity, the mechanism for protection and its induction. Such data will be invaluable for assessing the protective response as an exploitable mechanism for maintaining the general well-being of cells.